This section defines the syntax and semantics of all standard
HTTP/1.1 header fields. For entity-header fields, both sender and
recipient refer to either the client or the server, depending on who
sends and who receives the entity.

The Accept request-header field can be used to specify certain media
types which are acceptable for the response. Accept headers can be
used to indicate that the request is specifically limited to a small
set of desired types, as in the case of a request for an in-line
image.

The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The media-range MAY include media type
parameters that are applicable to that range.

Each media-range MAY be followed by one or more accept-params,
beginning with the "q" parameter for indicating a relative quality
factor. The first "q" parameter (if any) separates the media-range
parameter(s) from the accept-params. Quality factors allow the user
or user agent to indicate the relative degree of preference for that
media-range, using the qvalue scale from 0 to 1 (section 3.9). The
default value is q=1.

Note: Use of the "q" parameter name to separate media type
parameters from Accept extension parameters is due to historical
practice. Although this prevents any media type parameter named
"q" from being used with a media range, such an event is believed
to be unlikely given the lack of any "q" parameters in the IANA
media type registry and the rare usage of any media type
parameters in Accept. Future media types are discouraged from
registering any parameter named "q".

The example

Accept: audio/*; q=0.2, audio/basic

SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality."

If no Accept header field is present, then it is assumed that the
client accepts all media types. If an Accept header field is present,
and if the server cannot send a response which is acceptable
according to the combined Accept field value, then the server SHOULD
send a 406 (not acceptable) response.

A more elaborate example is

Accept: text/plain; q=0.5, text/html,
text/x-dvi; q=0.8, text/x-c

Verbally, this would be interpreted as "text/html and text/x-c are
the preferred media types, but if they do not exist, then send the
text/x-dvi entity, and if that does not exist, send the text/plain
entity."

Media ranges can be overridden by more specific media ranges or
specific media types. If more than one media range applies to a given
type, the most specific reference has precedence. For example,

Accept: text/*, text/html, text/html;level=1, */*

have the following precedence:

1) text/html;level=1
2) text/html
3) text/*
4) */*

The media type quality factor associated with a given type is
determined by finding the media range with the highest precedence
which matches that type. For example,

Note: A user agent might be provided with a default set of quality
values for certain media ranges. However, unless the user agent is
a closed system which cannot interact with other rendering agents,
this default set ought to be configurable by the user.

The Accept-Charset request-header field can be used to indicate what
character sets are acceptable for the response. This field allows
clients capable of understanding more comprehensive or special-
purpose character sets to signal that capability to a server which is
capable of representing documents in those character sets.

Character set values are described in section 3.4. Each charset MAY
be given an associated quality value which represents the user's
preference for that charset. The default value is q=1. An example is

Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

The special value "*", if present in the Accept-Charset field,
matches every character set (including ISO-8859-1) which is not
mentioned elsewhere in the Accept-Charset field. If no "*" is present
in an Accept-Charset field, then all character sets not explicitly
mentioned get a quality value of 0, except for ISO-8859-1, which gets
a quality value of 1 if not explicitly mentioned.

If no Accept-Charset header is present, the default is that any
character set is acceptable. If an Accept-Charset header is present,
and if the server cannot send a response which is acceptable
according to the Accept-Charset header, then the server SHOULD send
an error response with the 406 (not acceptable) status code, though
the sending of an unacceptable response is also allowed.

A server tests whether a content-coding is acceptable, according to
an Accept-Encoding field, using these rules:

1. If the content-coding is one of the content-codings listed in
the Accept-Encoding field, then it is acceptable, unless it is
accompanied by a qvalue of 0. (As defined in section 3.9, a
qvalue of 0 means "not acceptable.")

2. The special "*" symbol in an Accept-Encoding field matches any
available content-coding not explicitly listed in the header
field.

3. If multiple content-codings are acceptable, then the acceptable
content-coding with the highest non-zero qvalue is preferred.

4. The "identity" content-coding is always acceptable, unless
specifically refused because the Accept-Encoding field includes
"identity;q=0", or because the field includes "*;q=0" and does
not explicitly include the "identity" content-coding. If the
Accept-Encoding field-value is empty, then only the "identity"
encoding is acceptable.

If an Accept-Encoding field is present in a request, and if the
server cannot send a response which is acceptable according to the
Accept-Encoding header, then the server SHOULD send an error response
with the 406 (Not Acceptable) status code.

If no Accept-Encoding field is present in a request, the server MAY
assume that the client will accept any content coding. In this case,
if "identity" is one of the available content-codings, then the
server SHOULD use the "identity" content-coding, unless it has
additional information that a different content-coding is meaningful
to the client.

Note: If the request does not include an Accept-Encoding field,
and if the "identity" content-coding is unavailable, then
content-codings commonly understood by HTTP/1.0 clients (i.e.,

"gzip" and "compress") are preferred; some older clients
improperly display messages sent with other content-codings. The
server might also make this decision based on information about
the particular user-agent or client.

Note: Most HTTP/1.0 applications do not recognize or obey qvalues
associated with content-codings. This means that qvalues will not
work and are not permitted with x-gzip or x-compress.

Each language-range MAY be given an associated quality value which
represents an estimate of the user's preference for the languages
specified by that range. The quality value defaults to "q=1". For
example,

Accept-Language: da, en-gb;q=0.8, en;q=0.7

would mean: "I prefer Danish, but will accept British English and
other types of English." A language-range matches a language-tag if
it exactly equals the tag, or if it exactly equals a prefix of the
tag such that the first tag character following the prefix is "-".
The special range "*", if present in the Accept-Language field,
matches every tag not matched by any other range present in the
Accept-Language field.

Note: This use of a prefix matching rule does not imply that
language tags are assigned to languages in such a way that it is
always true that if a user understands a language with a certain
tag, then this user will also understand all languages with tags
for which this tag is a prefix. The prefix rule simply allows the
use of prefix tags if this is the case.

The language quality factor assigned to a language-tag by the
Accept-Language field is the quality value of the longest language-
range in the field that matches the language-tag. If no language-
range in the field matches the tag, the language quality factor
assigned is 0. If no Accept-Language header is present in the
request, the server

SHOULD assume that all languages are equally acceptable. If an
Accept-Language header is present, then all languages which are
assigned a quality factor greater than 0 are acceptable.

It might be contrary to the privacy expectations of the user to send
an Accept-Language header with the complete linguistic preferences of
the user in every request. For a discussion of this issue, see
section 15.1.4.

As intelligibility is highly dependent on the individual user, it is
recommended that client applications make the choice of linguistic
preference available to the user. If the choice is not made
available, then the Accept-Language header field MUST NOT be given in
the request.

Note: When making the choice of linguistic preference available to
the user, we remind implementors of the fact that users are not
familiar with the details of language matching as described above,
and should provide appropriate guidance. As an example, users
might assume that on selecting "en-gb", they will be served any
kind of English document if British English is not available. A
user agent might suggest in such a case to add "en" to get the
best matching behavior.

The Age response-header field conveys the sender's estimate of the
amount of time since the response (or its revalidation) was
generated at the origin server. A cached response is "fresh" if
its age does not exceed its freshness lifetime. Age values are
calculated as specified in section 13.2.3.

Age = "Age" ":" age-value
age-value = delta-seconds

Age values are non-negative decimal integers, representing time in
seconds.

If a cache receives a value larger than the largest positive
integer it can represent, or if any of its age calculations
overflows, it MUST transmit an Age header with a value of
2147483648 (2^31). An HTTP/1.1 server that includes a cache MUST
include an Age header field in every response generated from its
own cache. Caches SHOULD use an arithmetic type of at least 31
bits of range.

The Allow entity-header field lists the set of methods supported
by the resource identified by the Request-URI. The purpose of this
field is strictly to inform the recipient of valid methods
associated with the resource. An Allow header field MUST be
present in a 405 (Method Not Allowed) response.

Allow = "Allow" ":" #Method

Example of use:

Allow: GET, HEAD, PUT

This field cannot prevent a client from trying other methods.
However, the indications given by the Allow header field value
SHOULD be followed. The actual set of allowed methods is defined
by the origin server at the time of each request.

The Allow header field MAY be provided with a PUT request to
recommend the methods to be supported by the new or modified
resource. The server is not required to support these methods and
SHOULD include an Allow header in the response giving the actual
supported methods.

A proxy MUST NOT modify the Allow header field even if it does not
understand all the methods specified, since the user agent might
have other means of communicating with the origin server.

A user agent that wishes to authenticate itself with a server--
usually, but not necessarily, after receiving a 401 response--does
so by including an Authorization request-header field with the
request. The Authorization field value consists of credentials
containing the authentication information of the user agent for
the realm of the resource being requested.

Authorization = "Authorization" ":" credentials

HTTP access authentication is described in "HTTP Authentication:
Basic and Digest Access Authentication" [43]. If a request is
authenticated and a realm specified, the same credentials SHOULD
be valid for all other requests within this realm (assuming that
the authentication scheme itself does not require otherwise, such
as credentials that vary according to a challenge value or using
synchronized clocks).

When a shared cache (see section 13.7) receives a request
containing an Authorization field, it MUST NOT return the
corresponding response as a reply to any other request, unless one
of the following specific exceptions holds:

1. If the response includes the "s-maxage" cache-control
directive, the cache MAY use that response in replying to a
subsequent request. But (if the specified maximum age has
passed) a proxy cache MUST first revalidate it with the origin
server, using the request-headers from the new request to allow
the origin server to authenticate the new request. (This is the
defined behavior for s-maxage.) If the response includes "s-
maxage=0", the proxy MUST always revalidate it before re-using
it.

2. If the response includes the "must-revalidate" cache-control
directive, the cache MAY use that response in replying to a
subsequent request. But if the response is stale, all caches
MUST first revalidate it with the origin server, using the
request-headers from the new request to allow the origin server
to authenticate the new request.

3. If the response includes the "public" cache-control directive,
it MAY be returned in reply to any subsequent request.

The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response chain. The directives specify behavior intended to
prevent caches from adversely interfering with the request or
response. These directives typically override the default caching
algorithms. Cache directives are unidirectional in that the presence
of a directive in a request does not imply that the same directive is
to be given in the response.

Cache directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-
directive for a specific cache.

When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a
directive appears with a 1#field-name parameter, it applies only to
the named field or fields, and not to the rest of the request or
response. This mechanism supports extensibility; implementations of
future versions of the HTTP protocol might apply these directives to
header fields not defined in HTTP/1.1.

The cache-control directives can be broken down into these general
categories:

- Restrictions on what are cacheable; these may only be imposed by
the origin server.

- Restrictions on what may be stored by a cache; these may be
imposed by either the origin server or the user agent.

- Modifications of the basic expiration mechanism; these may be
imposed by either the origin server or the user agent.

- Controls over cache revalidation and reload; these may only be
imposed by a user agent.

By default, a response is cacheable if the requirements of the
request method, request header fields, and the response status
indicate that it is cacheable. Section 13.4 summarizes these defaults
for cacheability. The following Cache-Control response directives
allow an origin server to override the default cacheability of a
response:

public

Indicates that the response MAY be cached by any cache, even if it
would normally be non-cacheable or cacheable only within a non-
shared cache. (See also Authorization, section 14.8, for
additional details.)

private

Indicates that all or part of the response message is intended for
a single user and MUST NOT be cached by a shared cache. This
allows an origin server to state that the specified parts of the

response are intended for only one user and are not a valid
response for requests by other users. A private (non-shared) cache
MAY cache the response.

Note: This usage of the word private only controls where the
response may be cached, and cannot ensure the privacy of the
message content.

no-cache

If the no-cache directive does not specify a field-name, then a
cache MUST NOT use the response to satisfy a subsequent request
without successful revalidation with the origin server. This
allows an origin server to prevent caching even by caches that
have been configured to return stale responses to client requests.

If the no-cache directive does specify one or more field-names,
then a cache MAY use the response to satisfy a subsequent request,
subject to any other restrictions on caching. However, the
specified field-name(s) MUST NOT be sent in the response to a
subsequent request without successful revalidation with the origin
server. This allows an origin server to prevent the re-use of
certain header fields in a response, while still allowing caching
of the rest of the response.

The purpose of the no-store directive is to prevent the
inadvertent release or retention of sensitive information (for
example, on backup tapes). The no-store directive applies to the
entire message, and MAY be sent either in a response or in a
request. If sent in a request, a cache MUST NOT store any part of
either this request or any response to it. If sent in a response,
a cache MUST NOT store any part of either this response or the
request that elicited it. This directive applies to both non-
shared and shared caches. "MUST NOT store" in this context means
that the cache MUST NOT intentionally store the information in
non-volatile storage, and MUST make a best-effort attempt to
remove the information from volatile storage as promptly as
possible after forwarding it.

Even when this directive is associated with a response, users
might explicitly store such a response outside of the caching
system (e.g., with a "Save As" dialog). History buffers MAY store
such responses as part of their normal operation.

The purpose of this directive is to meet the stated requirements
of certain users and service authors who are concerned about
accidental releases of information via unanticipated accesses to
cache data structures. While the use of this directive might
improve privacy in some cases, we caution that it is NOT in any
way a reliable or sufficient mechanism for ensuring privacy. In
particular, malicious or compromised caches might not recognize or
obey this directive, and communications networks might be
vulnerable to eavesdropping.

The expiration time of an entity MAY be specified by the origin
server using the Expires header (see section 14.21). Alternatively,
it MAY be specified using the max-age directive in a response. When
the max-age cache-control directive is present in a cached response,
the response is stale if its current age is greater than the age
value given (in seconds) at the time of a new request for that
resource. The max-age directive on a response implies that the
response is cacheable (i.e., "public") unless some other, more
restrictive cache directive is also present.

If a response includes both an Expires header and a max-age
directive, the max-age directive overrides the Expires header, even
if the Expires header is more restrictive. This rule allows an origin
server to provide, for a given response, a longer expiration time to
an HTTP/1.1 (or later) cache than to an HTTP/1.0 cache. This might be
useful if certain HTTP/1.0 caches improperly calculate ages or
expiration times, perhaps due to desynchronized clocks.

Many HTTP/1.0 cache implementations will treat an Expires value that
is less than or equal to the response Date value as being equivalent
to the Cache-Control response directive "no-cache". If an HTTP/1.1
cache receives such a response, and the response does not include a
Cache-Control header field, it SHOULD consider the response to be
non-cacheable in order to retain compatibility with HTTP/1.0 servers.

Note: An origin server might wish to use a relatively new HTTP
cache control feature, such as the "private" directive, on a
network including older caches that do not understand that
feature. The origin server will need to combine the new feature
with an Expires field whose value is less than or equal to the
Date value. This will prevent older caches from improperly
caching the response.

s-maxage

If a response includes an s-maxage directive, then for a shared
cache (but not for a private cache), the maximum age specified by
this directive overrides the maximum age specified by either the
max-age directive or the Expires header. The s-maxage directive
also implies the semantics of the proxy-revalidate directive (see
section 14.9.4), i.e., that the shared cache must not use the
entry after it becomes stale to respond to a subsequent request
without first revalidating it with the origin server. The s-
maxage directive is always ignored by a private cache.

Note that most older caches, not compliant with this specification,
do not implement any cache-control directives. An origin server
wishing to use a cache-control directive that restricts, but does not
prevent, caching by an HTTP/1.1-compliant cache MAY exploit the
requirement that the max-age directive overrides the Expires header,
and the fact that pre-HTTP/1.1-compliant caches do not observe the
max-age directive.

Other directives allow a user agent to modify the basic expiration
mechanism. These directives MAY be specified on a request:

max-age

Indicates that the client is willing to accept a response whose
age is no greater than the specified time in seconds. Unless max-
stale directive is also included, the client is not willing to
accept a stale response.

min-fresh

Indicates that the client is willing to accept a response whose
freshness lifetime is no less than its current age plus the
specified time in seconds. That is, the client wants a response
that will still be fresh for at least the specified number of
seconds.

max-stale

Indicates that the client is willing to accept a response that has
exceeded its expiration time. If max-stale is assigned a value,
then the client is willing to accept a response that has exceeded
its expiration time by no more than the specified number of
seconds. If no value is assigned to max-stale, then the client is
willing to accept a stale response of any age.

If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to
override the expiration time of a response, the cache MUST attach a
Warning header to the stale response, using Warning 110 (Response is
stale).

A cache MAY be configured to return stale responses without
validation, but only if this does not conflict with any "MUST"-level
requirements concerning cache validation (e.g., a "must-revalidate"
cache-control directive).

If both the new request and the cached entry include "max-age"
directives, then the lesser of the two values is used for determining
the freshness of the cached entry for that request.

Sometimes a user agent might want or need to insist that a cache
revalidate its cache entry with the origin server (and not just with
the next cache along the path to the origin server), or to reload its
cache entry from the origin server. End-to-end revalidation might be
necessary if either the cache or the origin server has overestimated
the expiration time of the cached response. End-to-end reload may be
necessary if the cache entry has become corrupted for some reason.

End-to-end revalidation may be requested either when the client does
not have its own local cached copy, in which case we call it
"unspecified end-to-end revalidation", or when the client does have a
local cached copy, in which case we call it "specific end-to-end
revalidation."

The client can specify these three kinds of action using Cache-
Control request directives:

End-to-end reload

The request includes a "no-cache" cache-control directive or, for
compatibility with HTTP/1.0 clients, "Pragma: no-cache". Field
names MUST NOT be included with the no-cache directive in a
request. The server MUST NOT use a cached copy when responding to
such a request.

Specific end-to-end revalidation

The request includes a "max-age=0" cache-control directive, which
forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server.
The initial request includes a cache-validating conditional with
the client's current validator.

Unspecified end-to-end revalidation

The request includes "max-age=0" cache-control directive, which
forces each cache along the path to the origin server to
revalidate its own entry, if any, with the next cache or server.
The initial request does not include a cache-validating

conditional; the first cache along the path (if any) that holds a
cache entry for this resource includes a cache-validating
conditional with its current validator.

max-age

When an intermediate cache is forced, by means of a max-age=0
directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator
might differ from the validator currently stored with the cache
entry. In this case, the cache MAY use either validator in making
its own request without affecting semantic transparency.

However, the choice of validator might affect performance. The
best approach is for the intermediate cache to use its own
validator when making its request. If the server replies with 304
(Not Modified), then the cache can return its now validated copy
to the client with a 200 (OK) response. If the server replies with
a new entity and cache validator, however, the intermediate cache
can compare the returned validator with the one provided in the
client's request, using the strong comparison function. If the
client's validator is equal to the origin server's, then the
intermediate cache simply returns 304 (Not Modified). Otherwise,
it returns the new entity with a 200 (OK) response.

If a request includes the no-cache directive, it SHOULD NOT
include min-fresh, max-stale, or max-age.

only-if-cached

In some cases, such as times of extremely poor network
connectivity, a client may want a cache to return only those
responses that it currently has stored, and not to reload or
revalidate with the origin server. To do this, the client may
include the only-if-cached directive in a request. If it receives
this directive, a cache SHOULD either respond using a cached entry
that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status. However, if a group
of caches is being operated as a unified system with good internal
connectivity, such a request MAY be forwarded within that group of
caches.

must-revalidate

Because a cache MAY be configured to ignore a server's specified
expiration time, and because a client request MAY include a max-
stale directive (which has a similar effect), the protocol also
includes a mechanism for the origin server to require revalidation
of a cache entry on any subsequent use. When the must-revalidate
directive is present in a response received by a cache, that cache
MUST NOT use the entry after it becomes stale to respond to a

subsequent request without first revalidating it with the origin
server. (I.e., the cache MUST do an end-to-end revalidation every
time, if, based solely on the origin server's Expires or max-age
value, the cached response is stale.)

The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances an
HTTP/1.1 cache MUST obey the must-revalidate directive; in
particular, if the cache cannot reach the origin server for any
reason, it MUST generate a 504 (Gateway Timeout) response.

Servers SHOULD send the must-revalidate directive if and only if
failure to revalidate a request on the entity could result in
incorrect operation, such as a silently unexecuted financial
transaction. Recipients MUST NOT take any automated action that
violates this directive, and MUST NOT automatically provide an
unvalidated copy of the entity if revalidation fails.

Although this is not recommended, user agents operating under
severe connectivity constraints MAY violate this directive but, if
so, MUST explicitly warn the user that an unvalidated response has
been provided. The warning MUST be provided on each unvalidated
access, and SHOULD require explicit user confirmation.

proxy-revalidate

The proxy-revalidate directive has the same meaning as the must-
revalidate directive, except that it does not apply to non-shared
user agent caches. It can be used on a response to an
authenticated request to permit the user's cache to store and
later return the response without needing to revalidate it (since
it has already been authenticated once by that user), while still
requiring proxies that service many users to revalidate each time
(in order to make sure that each user has been authenticated).
Note that such authenticated responses also need the public cache
control directive in order to allow them to be cached at all.

Implementors of intermediate caches (proxies) have found it useful
to convert the media type of certain entity bodies. A non-
transparent proxy might, for example, convert between image
formats in order to save cache space or to reduce the amount of
traffic on a slow link.

Serious operational problems occur, however, when these
transformations are applied to entity bodies intended for certain
kinds of applications. For example, applications for medical

imaging, scientific data analysis and those using end-to-end
authentication, all depend on receiving an entity body that is bit
for bit identical to the original entity-body.

Therefore, if a message includes the no-transform directive, an
intermediate cache or proxy MUST NOT change those headers that are
listed in section 13.5.2 as being subject to the no-transform
directive. This implies that the cache or proxy MUST NOT change
any aspect of the entity-body that is specified by these headers,
including the value of the entity-body itself.

The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional assigned value.
Informational extensions (those which do not require a change in
cache behavior) MAY be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new
directive and the standard directive are supplied, such that
applications which do not understand the new directive will default
to the behavior specified by the standard directive, and those that
understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without
requiring changes to the base protocol.

This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.

For example, consider a hypothetical new response directive called
community which acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any non-shared
cache, any cache which is shared only by members of the community
named within its value may cache the response. An origin server
wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including

Cache-Control: private, community="UCI"

A cache seeing this header field will act correctly even if the cache
does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe
behavior.

Unrecognized cache-directives MUST be ignored; it is assumed that any
cache-directive likely to be unrecognized by an HTTP/1.1 cache will
be combined with standard directives (or the response's default
cacheability) such that the cache behavior will remain minimally
correct even if the cache does not understand the extension(s).

HTTP/1.1 proxies MUST parse the Connection header field before a
message is forwarded and, for each connection-token in this field,
remove any header field(s) from the message with the same name as the
connection-token. Connection options are signaled by the presence of
a connection-token in the Connection header field, not by any
corresponding additional header field(s), since the additional header
field may not be sent if there are no parameters associated with that
connection option.

Message headers listed in the Connection header MUST NOT include
end-to-end headers, such as Cache-Control.

HTTP/1.1 defines the "close" connection option for the sender to
signal that the connection will be closed after completion of the
response. For example,

Connection: close

in either the request or the response header fields indicates that
the connection SHOULD NOT be considered `persistent' (section 8.1)
after the current request/response is complete.

HTTP/1.1 applications that do not support persistent connections MUST
include the "close" connection option in every message.

A system receiving an HTTP/1.0 (or lower-version) message that
includes a Connection header MUST, for each connection-token in this
field, remove and ignore any header field(s) from the message with
the same name as the connection-token. This protects against mistaken
forwarding of such header fields by pre-HTTP/1.1 proxies. See section
19.6.2.

The Content-Encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional content
codings have been applied to the entity-body, and thus what decoding
mechanisms must be applied in order to obtain the media-type
referenced by the Content-Type header field. Content-Encoding is
primarily used to allow a document to be compressed without losing
the identity of its underlying media type.

The content-coding is a characteristic of the entity identified by
the Request-URI. Typically, the entity-body is stored with this
encoding and is only decoded before rendering or analogous usage.
However, a non-transparent proxy MAY modify the content-coding if the
new coding is known to be acceptable to the recipient, unless the
"no-transform" cache-control directive is present in the message.

If the content-coding of an entity is not "identity", then the
response MUST include a Content-Encoding entity-header (section
14.11) that lists the non-identity content-coding(s) used.

If the content-coding of an entity in a request message is not
acceptable to the origin server, the server SHOULD respond with a
status code of 415 (Unsupported Media Type).

If multiple encodings have been applied to an entity, the content
codings MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided
by other entity-header fields not defined by this specification.

The Content-Language entity-header field describes the natural
language(s) of the intended audience for the enclosed entity. Note
that this might not be equivalent to all the languages used within
the entity-body.

Content-Language = "Content-Language" ":" 1#language-tag

Language tags are defined in section 3.10. The primary purpose of
Content-Language is to allow a user to identify and differentiate
entities according to the user's own preferred language. Thus, if the
body content is intended only for a Danish-literate audience, the
appropriate field is

Content-Language: da

If no Content-Language is specified, the default is that the content
is intended for all language audiences. This might mean that the
sender does not consider it to be specific to any natural language,
or that the sender does not know for which language it is intended.

Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi," presented simultaneously in the original Maori and English
versions, would call for

Content-Language: mi, en

However, just because multiple languages are present within an entity
does not mean that it is intended for multiple linguistic audiences.
An example would be a beginner's language primer, such as "A First
Lesson in Latin," which is clearly intended to be used by an
English-literate audience. In this case, the Content-Language would
properly only include "en".

Content-Language MAY be applied to any media type -- it is not
limited to textual documents.

The Content-Length entity-header field indicates the size of the
entity-body, in decimal number of OCTETs, sent to the recipient or,
in the case of the HEAD method, the size of the entity-body that
would have been sent had the request been a GET.

Content-Length = "Content-Length" ":" 1*DIGIT

An example is

Content-Length: 3495

Applications SHOULD use this field to indicate the transfer-length of
the message-body, unless this is prohibited by the rules in section
4.4.

Any Content-Length greater than or equal to zero is a valid value.
Section 4.4 describes how to determine the length of a message-body
if a Content-Length is not given.

Note that the meaning of this field is significantly different from
the corresponding definition in MIME, where it is an optional field
used within the "message/external-body" content-type. In HTTP, it
SHOULD be sent whenever the message's length can be determined prior
to being transferred, unless this is prohibited by the rules in
section 4.4.

The Content-Location entity-header field MAY be used to supply the
resource location for the entity enclosed in the message when that
entity is accessible from a location separate from the requested
resource's URI. A server SHOULD provide a Content-Location for the
variant corresponding to the response entity; especially in the case
where a resource has multiple entities associated with it, and those
entities actually have separate locations by which they might be
individually accessed, the server SHOULD provide a Content-Location
for the particular variant which is returned.

The value of Content-Location also defines the base URI for the
entity.

The Content-Location value is not a replacement for the original
requested URI; it is only a statement of the location of the resource
corresponding to this particular entity at the time of the request.
Future requests MAY specify the Content-Location URI as the request-
URI if the desire is to identify the source of that particular
entity.

A cache cannot assume that an entity with a Content-Location
different from the URI used to retrieve it can be used to respond to
later requests on that Content-Location URI. However, the Content-
Location can be used to differentiate between multiple entities
retrieved from a single requested resource, as described in section
13.6.

If the Content-Location is a relative URI, the relative URI is
interpreted relative to the Request-URI.

The meaning of the Content-Location header in PUT or POST requests is
undefined; servers are free to ignore it in those cases.

The Content-MD5 entity-header field, as defined in RFC 1864 [23], is
an MD5 digest of the entity-body for the purpose of providing an
end-to-end message integrity check (MIC) of the entity-body. (Note: a
MIC is good for detecting accidental modification of the entity-body
in transit, but is not proof against malicious attacks.)

The Content-MD5 header field MAY be generated by an origin server or
client to function as an integrity check of the entity-body. Only
origin servers or clients MAY generate the Content-MD5 header field;
proxies and gateways MUST NOT generate it, as this would defeat its
value as an end-to-end integrity check. Any recipient of the entity-
body, including gateways and proxies, MAY check that the digest value
in this header field matches that of the entity-body as received.

The MD5 digest is computed based on the content of the entity-body,
including any content-coding that has been applied, but not including
any transfer-encoding applied to the message-body. If the message is
received with a transfer-encoding, that encoding MUST be removed
prior to checking the Content-MD5 value against the received entity.

This has the result that the digest is computed on the octets of the
entity-body exactly as, and in the order that, they would be sent if
no transfer-encoding were being applied.

HTTP extends RFC 1864 to permit the digest to be computed for MIME
composite media-types (e.g., multipart/* and message/rfc822), but
this does not change how the digest is computed as defined in the
preceding paragraph.

There are several consequences of this. The entity-body for composite
types MAY contain many body-parts, each with its own MIME and HTTP
headers (including Content-MD5, Content-Transfer-Encoding, and
Content-Encoding headers). If a body-part has a Content-Transfer-
Encoding or Content-Encoding header, it is assumed that the content
of the body-part has had the encoding applied, and the body-part is
included in the Content-MD5 digest as is -- i.e., after the
application. The Transfer-Encoding header field is not allowed within
body-parts.

Conversion of all line breaks to CRLF MUST NOT be done before
computing or checking the digest: the line break convention used in
the text actually transmitted MUST be left unaltered when computing
the digest.

Note: while the definition of Content-MD5 is exactly the same for
HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
in which the application of Content-MD5 to HTTP entity-bodies
differs from its application to MIME entity-bodies. One is that
HTTP, unlike MIME, does not use Content-Transfer-Encoding, and
does use Transfer-Encoding and Content-Encoding. Another is that
HTTP more frequently uses binary content types than MIME, so it is
worth noting that, in such cases, the byte order used to compute
the digest is the transmission byte order defined for the type.
Lastly, HTTP allows transmission of text types with any of several
line break conventions and not just the canonical form using CRLF.

The header SHOULD indicate the total length of the full entity-body,
unless this length is unknown or difficult to determine. The asterisk
"*" character means that the instance-length is unknown at the time
when the response was generated.

Unlike byte-ranges-specifier values (see section 14.35.1), a byte-
range-resp-spec MUST only specify one range, and MUST contain
absolute byte positions for both the first and last byte of the
range.

A byte-content-range-spec with a byte-range-resp-spec whose last-
byte-pos value is less than its first-byte-pos value, or whose
instance-length value is less than or equal to its last-byte-pos
value, is invalid. The recipient of an invalid byte-content-range-
spec MUST ignore it and any content transferred along with it.

A server sending a response with status code 416 (Requested range not
satisfiable) SHOULD include a Content-Range field with a byte-range-
resp-spec of "*". The instance-length specifies the current length of

the selected resource. A response with status code 206 (Partial
Content) MUST NOT include a Content-Range field with a byte-range-
resp-spec of "*".

Examples of byte-content-range-spec values, assuming that the entity
contains a total of 1234 bytes:

. The first 500 bytes:
bytes 0-499/1234

. The second 500 bytes:
bytes 500-999/1234

. All except for the first 500 bytes:
bytes 500-1233/1234

. The last 500 bytes:
bytes 734-1233/1234

When an HTTP message includes the content of a single range (for
example, a response to a request for a single range, or to a request
for a set of ranges that overlap without any holes), this content is
transmitted with a Content-Range header, and a Content-Length header
showing the number of bytes actually transferred. For example,

When an HTTP message includes the content of multiple ranges (for
example, a response to a request for multiple non-overlapping
ranges), these are transmitted as a multipart message. The multipart
media type used for this purpose is "multipart/byteranges" as defined
in appendix 19.2. See appendix 19.6.3 for a compatibility issue.

A response to a request for a single range MUST NOT be sent using the
multipart/byteranges media type. A response to a request for
multiple ranges, whose result is a single range, MAY be sent as a
multipart/byteranges media type with one part. A client that cannot
decode a multipart/byteranges message MUST NOT ask for multiple
byte-ranges in a single request.

When a client requests multiple byte-ranges in one request, the
server SHOULD return them in the order that they appeared in the
request.

If the server ignores a byte-range-spec because it is syntactically
invalid, the server SHOULD treat the request as if the invalid Range
header field did not exist. (Normally, this means return a 200
response containing the full entity).

If the server receives a request (other than one including an If-
Range request-header field) with an unsatisfiable Range request-
header field (that is, all of whose byte-range-spec values have a
first-byte-pos value greater than the current length of the selected
resource), it SHOULD return a response code of 416 (Requested range
not satisfiable) (section 10.4.17).

Note: clients cannot depend on servers to send a 416 (Requested
range not satisfiable) response instead of a 200 (OK) response for
an unsatisfiable Range request-header, since not all servers
implement this request-header.

The Content-Type entity-header field indicates the media type of the
entity-body sent to the recipient or, in the case of the HEAD method,
the media type that would have been sent had the request been a GET.

The Date general-header field represents the date and time at which
the message was originated, having the same semantics as orig-date in
RFC 822. The field value is an HTTP-date, as described in section
3.3.1; it MUST be sent in RFC 1123 [8]-date format.

Date = "Date" ":" HTTP-date

An example is

Date: Tue, 15 Nov 1994 08:12:31 GMT

Origin servers MUST include a Date header field in all responses,
except in these cases:

1. If the response status code is 100 (Continue) or 101 (Switching
Protocols), the response MAY include a Date header field, at
the server's option.

2. If the response status code conveys a server error, e.g. 500
(Internal Server Error) or 503 (Service Unavailable), and it is
inconvenient or impossible to generate a valid Date.

3. If the server does not have a clock that can provide a
reasonable approximation of the current time, its responses
MUST NOT include a Date header field. In this case, the rules
in section 14.18.1 MUST be followed.

A received message that does not have a Date header field MUST be
assigned one by the recipient if the message will be cached by that
recipient or gatewayed via a protocol which requires a Date. An HTTP
implementation without a clock MUST NOT cache responses without
revalidating them on every use. An HTTP cache, especially a shared
cache, SHOULD use a mechanism, such as NTP [28], to synchronize its
clock with a reliable external standard.

Clients SHOULD only send a Date header field in messages that include
an entity-body, as in the case of the PUT and POST requests, and even
then it is optional. A client without a clock MUST NOT send a Date
header field in a request.

The HTTP-date sent in a Date header SHOULD NOT represent a date and
time subsequent to the generation of the message. It SHOULD represent
the best available approximation of the date and time of message
generation, unless the implementation has no means of generating a
reasonably accurate date and time. In theory, the date ought to
represent the moment just before the entity is generated. In
practice, the date can be generated at any time during the message
origination without affecting its semantic value.

Some origin server implementations might not have a clock available.
An origin server without a clock MUST NOT assign Expires or Last-
Modified values to a response, unless these values were associated
with the resource by a system or user with a reliable clock. It MAY
assign an Expires value that is known, at or before server
configuration time, to be in the past (this allows "pre-expiration"
of responses without storing separate Expires values for each
resource).

The ETag response-header field provides the current value of the
entity tag for the requested variant. The headers used with entity
tags are described in sections 14.24, 14.26 and 14.44. The entity tag
MAY be used for comparison with other entities from the same resource
(see section 13.3.3).

A server that does not understand or is unable to comply with any of
the expectation values in the Expect field of a request MUST respond
with appropriate error status. The server MUST respond with a 417
(Expectation Failed) status if any of the expectations cannot be met
or, if there are other problems with the request, some other 4xx
status.

This header field is defined with extensible syntax to allow for
future extensions. If a server receives a request containing an
Expect field that includes an expectation-extension that it does not
support, it MUST respond with a 417 (Expectation Failed) status.

Comparison of expectation values is case-insensitive for unquoted
tokens (including the 100-continue token), and is case-sensitive for
quoted-string expectation-extensions.

The Expect mechanism is hop-by-hop: that is, an HTTP/1.1 proxy MUST
return a 417 (Expectation Failed) status if it receives a request
with an expectation that it cannot meet. However, the Expect
request-header itself is end-to-end; it MUST be forwarded if the
request is forwarded.

Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
Expect header.

The Expires entity-header field gives the date/time after which the
response is considered stale. A stale cache entry may not normally be
returned by a cache (either a proxy cache or a user agent cache)
unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the entity). See section
13.2 for further discussion of the expiration model.

The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.

The format is an absolute date and time as defined by HTTP-date in
section 3.3.1; it MUST be in RFC 1123 date format:

Expires = "Expires" ":" HTTP-date

An example of its use is

Expires: Thu, 01 Dec 1994 16:00:00 GMT

Note: if a response includes a Cache-Control field with the max-
age directive (see section 14.9.3), that directive overrides the
Expires field.

HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").

To mark a response as "already expired," an origin server sends an
Expires date that is equal to the Date header value. (See the rules
for expiration calculations in section 13.2.4.)

To mark a response as "never expires," an origin server sends an
Expires date approximately one year from the time the response is
sent. HTTP/1.1 servers SHOULD NOT send Expires dates more than one
year in the future.

The presence of an Expires header field with a date value of some
time in the future on a response that otherwise would by default be
non-cacheable indicates that the response is cacheable, unless
indicated otherwise by a Cache-Control header field (section 14.9).

The From request-header field, if given, SHOULD contain an Internet
e-mail address for the human user who controls the requesting user
agent. The address SHOULD be machine-usable, as defined by "mailbox"
in RFC 822 [9] as updated by RFC 1123 [8]:

From = "From" ":" mailbox

An example is:

From: webmaster@w3.org

This header field MAY be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It SHOULD NOT
be used as an insecure form of access protection. The interpretation
of this field is that the request is being performed on behalf of the
person given, who accepts responsibility for the method performed. In
particular, robot agents SHOULD include this header so that the
person responsible for running the robot can be contacted if problems
occur on the receiving end.

The Internet e-mail address in this field MAY be separate from the
Internet host which issued the request. For example, when a request
is passed through a proxy the original issuer's address SHOULD be
used.

The client SHOULD NOT send the From header field without the user's
approval, as it might conflict with the user's privacy interests or
their site's security policy. It is strongly recommended that the
user be able to disable, enable, and modify the value of this field
at any time prior to a request.

The Host request-header field specifies the Internet host and port
number of the resource being requested, as obtained from the original
URI given by the user or referring resource (generally an HTTP URL,

as described in section 3.2.2). The Host field value MUST represent
the naming authority of the origin server or gateway given by the
original URL. This allows the origin server or gateway to
differentiate between internally-ambiguous URLs, such as the root "/"
URL of a server for multiple host names on a single IP address.

Host = "Host" ":" host [ ":" port ] ; Section 3.2.2

A "host" without any trailing port information implies the default
port for the service requested (e.g., "80" for an HTTP URL). For
example, a request on the origin server for
<http://www.w3.org/pub/WWW/> would properly include:

GET /pub/WWW/ HTTP/1.1
Host: www.w3.org

A client MUST include a Host header field in all HTTP/1.1 request
messages . If the requested URI does not include an Internet host
name for the service being requested, then the Host header field MUST
be given with an empty value. An HTTP/1.1 proxy MUST ensure that any
request message it forwards does contain an appropriate Host header
field that identifies the service being requested by the proxy. All
Internet-based HTTP/1.1 servers MUST respond with a 400 (Bad Request)
status code to any HTTP/1.1 request message which lacks a Host header
field.

See sections 5.2 and 19.6.1.1 for other requirements relating to
Host.

The If-Match request-header field is used with a method to make it
conditional. A client that has one or more entities previously
obtained from the resource can verify that one of those entities is
current by including a list of their associated entity tags in the
If-Match header field. Entity tags are defined in section 3.11. The
purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead. It is also
used, on updating requests, to prevent inadvertent modification of
the wrong version of a resource. As a special case, the value "*"
matches any current entity of the resource.

If-Match = "If-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-Match header) on that resource, or if "*" is given

and any current entity exists for that resource, then the server MAY
perform the requested method as if the If-Match header field did not
exist.

A server MUST use the strong comparison function (see section 13.3.3)
to compare the entity tags in If-Match.

If none of the entity tags match, or if "*" is given and no current
entity exists, the server MUST NOT perform the requested method, and
MUST return a 412 (Precondition Failed) response. This behavior is
most useful when the client wants to prevent an updating method, such
as PUT, from modifying a resource that has changed since the client
last retrieved it.

If the request would, without the If-Match header field, result in
anything other than a 2xx or 412 status, then the If-Match header
MUST be ignored.

The meaning of "If-Match: *" is that the method SHOULD be performed
if the representation selected by the origin server (or by a cache,
possibly using the Vary mechanism, see section 14.44) exists, and
MUST NOT be performed if the representation does not exist.

A request intended to update a resource (e.g., a PUT) MAY include an
If-Match header field to signal that the request method MUST NOT be
applied if the entity corresponding to the If-Match value (a single
entity tag) is no longer a representation of that resource. This
allows the user to indicate that they do not wish the request to be
successful if the resource has been changed without their knowledge.
Examples:

The If-Modified-Since request-header field is used with a method to
make it conditional: if the requested variant has not been modified
since the time specified in this field, an entity will not be
returned from the server; instead, a 304 (not modified) response will
be returned without any message-body.

If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Since header and no Range header
requests that the identified entity be transferred only if it has
been modified since the date given by the If-Modified-Since header.
The algorithm for determining this includes the following cases:

a) If the request would normally result in anything other than a
200 (OK) status, or if the passed If-Modified-Since date is
invalid, the response is exactly the same as for a normal GET.
A date which is later than the server's current time is
invalid.

b) If the variant has been modified since the If-Modified-Since
date, the response is exactly the same as for a normal GET.

c) If the variant has not been modified since a valid If-
Modified-Since date, the server SHOULD return a 304 (Not
Modified) response.

The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.

Note: The Range request-header field modifies the meaning of If-
Modified-Since; see section 14.35 for full details.

Note: If-Modified-Since times are interpreted by the server, whose
clock might not be synchronized with the client.

Note: When handling an If-Modified-Since header field, some
servers will use an exact date comparison function, rather than a
less-than function, for deciding whether to send a 304 (Not
Modified) response. To get best results when sending an If-
Modified-Since header field for cache validation, clients are
advised to use the exact date string received in a previous Last-
Modified header field whenever possible.

Note: If a client uses an arbitrary date in the If-Modified-Since
header instead of a date taken from the Last-Modified header for
the same request, the client should be aware of the fact that this
date is interpreted in the server's understanding of time. The
client should consider unsynchronized clocks and rounding problems
due to the different encodings of time between the client and
server. This includes the possibility of race conditions if the
document has changed between the time it was first requested and
the If-Modified-Since date of a subsequent request, and the

possibility of clock-skew-related problems if the If-Modified-
Since date is derived from the client's clock without correction
to the server's clock. Corrections for different time bases
between client and server are at best approximate due to network
latency.

The result of a request having both an If-Modified-Since header field
and either an If-Match or an If-Unmodified-Since header fields is
undefined by this specification.

The If-None-Match request-header field is used with a method to make
it conditional. A client that has one or more entities previously
obtained from the resource can verify that none of those entities is
current by including a list of their associated entity tags in the
If-None-Match header field. The purpose of this feature is to allow
efficient updates of cached information with a minimum amount of
transaction overhead. It is also used to prevent a method (e.g. PUT)
from inadvertently modifying an existing resource when the client
believes that the resource does not exist.

As a special case, the value "*" matches any current entity of the
resource.

If-None-Match = "If-None-Match" ":" ( "*" | 1#entity-tag )

If any of the entity tags match the entity tag of the entity that
would have been returned in the response to a similar GET request
(without the If-None-Match header) on that resource, or if "*" is
given and any current entity exists for that resource, then the
server MUST NOT perform the requested method, unless required to do
so because the resource's modification date fails to match that
supplied in an If-Modified-Since header field in the request.
Instead, if the request method was GET or HEAD, the server SHOULD
respond with a 304 (Not Modified) response, including the cache-
related header fields (particularly ETag) of one of the entities that
matched. For all other request methods, the server MUST respond with
a status of 412 (Precondition Failed).

See section 13.3.3 for rules on how to determine if two entities tags
match. The weak comparison function can only be used with GET or HEAD
requests.

If none of the entity tags match, then the server MAY perform the
requested method as if the If-None-Match header field did not exist,
but MUST also ignore any If-Modified-Since header field(s) in the
request. That is, if no entity tags match, then the server MUST NOT
return a 304 (Not Modified) response.

If the request would, without the If-None-Match header field, result
in anything other than a 2xx or 304 status, then the If-None-Match
header MUST be ignored. (See section 13.3.4 for a discussion of
server behavior when both If-Modified-Since and If-None-Match appear
in the same request.)

The meaning of "If-None-Match: *" is that the method MUST NOT be
performed if the representation selected by the origin server (or by
a cache, possibly using the Vary mechanism, see section 14.44)
exists, and SHOULD be performed if the representation does not exist.
This feature is intended to be useful in preventing races between PUT
operations.

If a client has a partial copy of an entity in its cache, and wishes
to have an up-to-date copy of the entire entity in its cache, it
could use the Range request-header with a conditional GET (using
either or both of If-Unmodified-Since and If-Match.) However, if the
condition fails because the entity has been modified, the client
would then have to make a second request to obtain the entire current
entity-body.

The If-Range header allows a client to "short-circuit" the second
request. Informally, its meaning is `if the entity is unchanged, send
me the part(s) that I am missing; otherwise, send me the entire new
entity'.

If-Range = "If-Range" ":" ( entity-tag | HTTP-date )

If the client has no entity tag for an entity, but does have a Last-
Modified date, it MAY use that date in an If-Range header. (The
server can distinguish between a valid HTTP-date and any form of
entity-tag by examining no more than two characters.) The If-Range
header SHOULD only be used together with a Range header, and MUST be
ignored if the request does not include a Range header, or if the
server does not support the sub-range operation.

If the entity tag given in the If-Range header matches the current
entity tag for the entity, then the server SHOULD provide the
specified sub-range of the entity using a 206 (Partial content)
response. If the entity tag does not match, then the server SHOULD
return the entire entity using a 200 (OK) response.

The If-Unmodified-Since request-header field is used with a method to
make it conditional. If the requested resource has not been modified
since the time specified in this field, the server SHOULD perform the
requested operation as if the If-Unmodified-Since header were not
present.

If the requested variant has been modified since the specified time,
the server MUST NOT perform the requested operation, and MUST return
a 412 (Precondition Failed).

If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date

An example of the field is:

If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

If the request normally (i.e., without the If-Unmodified-Since
header) would result in anything other than a 2xx or 412 status, the
If-Unmodified-Since header SHOULD be ignored.

If the specified date is invalid, the header is ignored.

The result of a request having both an If-Unmodified-Since header
field and either an If-None-Match or an If-Modified-Since header
fields is undefined by this specification.

The Last-Modified entity-header field indicates the date and time at
which the origin server believes the variant was last modified.

Last-Modified = "Last-Modified" ":" HTTP-date

An example of its use is

Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

The exact meaning of this header field depends on the implementation
of the origin server and the nature of the original resource. For
files, it may be just the file system last-modified time. For
entities with dynamically included parts, it may be the most recent
of the set of last-modify times for its component parts. For database
gateways, it may be the last-update time stamp of the record. For
virtual objects, it may be the last time the internal state changed.

An origin server MUST NOT send a Last-Modified date which is later
than the server's time of message origination. In such cases, where
the resource's last modification would indicate some time in the
future, the server MUST replace that date with the message
origination date.

An origin server SHOULD obtain the Last-Modified value of the entity
as close as possible to the time that it generates the Date value of
its response. This allows a recipient to make an accurate assessment
of the entity's modification time, especially if the entity changes
near the time that the response is generated.

The Location response-header field is used to redirect the recipient
to a location other than the Request-URI for completion of the
request or identification of a new resource. For 201 (Created)
responses, the Location is that of the new resource which was created
by the request. For 3xx responses, the location SHOULD indicate the
server's preferred URI for automatic redirection to the resource. The
field value consists of a single absolute URI.

Location = "Location" ":" absoluteURI

An example is:

Location: http://www.w3.org/pub/WWW/People.html

Note: The Content-Location header field (section 14.14) differs
from Location in that the Content-Location identifies the original
location of the entity enclosed in the request. It is therefore
possible for a response to contain header fields for both Location
and Content-Location. Also see section 13.10 for cache
requirements of some methods.

The Max-Forwards request-header field provides a mechanism with the
TRACE (section 9.8) and OPTIONS (section 9.2) methods to limit the
number of proxies or gateways that can forward the request to the
next inbound server. This can be useful when the client is attempting
to trace a request chain which appears to be failing or looping in
mid-chain.

Max-Forwards = "Max-Forwards" ":" 1*DIGIT

The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message may be forwarded.

Each proxy or gateway recipient of a TRACE or OPTIONS request
containing a Max-Forwards header field MUST check and update its
value prior to forwarding the request. If the received value is zero
(0), the recipient MUST NOT forward the request; instead, it MUST
respond as the final recipient. If the received Max-Forwards value is
greater than zero, then the forwarded message MUST contain an updated
Max-Forwards field with a value decremented by one (1).

The Max-Forwards header field MAY be ignored for all other methods
defined by this specification and for any extension methods for which
it is not explicitly referred to as part of that method definition.

The Pragma general-header field is used to include implementation-
specific directives that might apply to any recipient along the
request/response chain. All pragma directives specify optional
behavior from the viewpoint of the protocol; however, some systems
MAY require that behavior be consistent with the directives.

When the no-cache directive is present in a request message, an
application SHOULD forward the request toward the origin server even
if it has a cached copy of what is being requested. This pragma
directive has the same semantics as the no-cache cache-directive (see
section 14.9) and is defined here for backward compatibility with
HTTP/1.0. Clients SHOULD include both header fields when a no-cache
request is sent to a server not known to be HTTP/1.1 compliant.

Pragma directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application,
since the directives might be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient.

HTTP/1.1 caches SHOULD treat "Pragma: no-cache" as if the client had
sent "Cache-Control: no-cache". No new Pragma directives will be
defined in HTTP.

Note: because the meaning of "Pragma: no-cache as a response
header field is not actually specified, it does not provide a
reliable replacement for "Cache-Control: no-cache" in a response

The Proxy-Authenticate response-header field MUST be included as part
of a 407 (Proxy Authentication Required) response. The field value
consists of a challenge that indicates the authentication scheme and
parameters applicable to the proxy for this Request-URI.

Proxy-Authenticate = "Proxy-Authenticate" ":" 1#challenge

The HTTP access authentication process is described in "HTTP
Authentication: Basic and Digest Access Authentication" [43]. Unlike
WWW-Authenticate, the Proxy-Authenticate header field applies only to
the current connection and SHOULD NOT be passed on to downstream
clients. However, an intermediate proxy might need to obtain its own
credentials by requesting them from the downstream client, which in
some circumstances will appear as if the proxy is forwarding the
Proxy-Authenticate header field.

The Proxy-Authorization request-header field allows the client to
identify itself (or its user) to a proxy which requires
authentication. The Proxy-Authorization field value consists of
credentials containing the authentication information of the user
agent for the proxy and/or realm of the resource being requested.

Proxy-Authorization = "Proxy-Authorization" ":" credentials

The HTTP access authentication process is described in "HTTP
Authentication: Basic and Digest Access Authentication" [43] . Unlike
Authorization, the Proxy-Authorization header field applies only to
the next outbound proxy that demanded authentication using the Proxy-
Authenticate field. When multiple proxies are used in a chain, the

Proxy-Authorization header field is consumed by the first outbound
proxy that was expecting to receive credentials. A proxy MAY relay
the credentials from the client request to the next proxy if that is
the mechanism by which the proxies cooperatively authenticate a given
request.

Since all HTTP entities are represented in HTTP messages as sequences
of bytes, the concept of a byte range is meaningful for any HTTP
entity. (However, not all clients and servers need to support byte-
range operations.)

Byte range specifications in HTTP apply to the sequence of bytes in
the entity-body (not necessarily the same as the message-body).

A byte range operation MAY specify a single range of bytes, or a set
of ranges within a single entity.

The first-byte-pos value in a byte-range-spec gives the byte-offset
of the first byte in a range. The last-byte-pos value gives the
byte-offset of the last byte in the range; that is, the byte
positions specified are inclusive. Byte offsets start at zero.

If the last-byte-pos value is present, it MUST be greater than or
equal to the first-byte-pos in that byte-range-spec, or the byte-
range-spec is syntactically invalid. The recipient of a byte-range-
set that includes one or more syntactically invalid byte-range-spec
values MUST ignore the header field that includes that byte-range-
set.

If the last-byte-pos value is absent, or if the value is greater than
or equal to the current length of the entity-body, last-byte-pos is
taken to be equal to one less than the current length of the entity-
body in bytes.

By its choice of last-byte-pos, a client can limit the number of
bytes retrieved without knowing the size of the entity.

suffix-byte-range-spec = "-" suffix-length
suffix-length = 1*DIGIT

A suffix-byte-range-spec is used to specify the suffix of the
entity-body, of a length given by the suffix-length value. (That is,
this form specifies the last N bytes of an entity-body.) If the
entity is shorter than the specified suffix-length, the entire
entity-body is used.

If a syntactically valid byte-range-set includes at least one byte-
range-spec whose first-byte-pos is less than the current length of
the entity-body, or at least one suffix-byte-range-spec with a non-
zero suffix-length, then the byte-range-set is satisfiable.
Otherwise, the byte-range-set is unsatisfiable. If the byte-range-set
is unsatisfiable, the server SHOULD return a response with a status
of 416 (Requested range not satisfiable). Otherwise, the server
SHOULD return a response with a status of 206 (Partial Content)
containing the satisfiable ranges of the entity-body.

Examples of byte-ranges-specifier values (assuming an entity-body of
length 10000):

HTTP retrieval requests using conditional or unconditional GET
methods MAY request one or more sub-ranges of the entity, instead of
the entire entity, using the Range request header, which applies to
the entity returned as the result of the request:

Range = "Range" ":" ranges-specifier

A server MAY ignore the Range header. However, HTTP/1.1 origin
servers and intermediate caches ought to support byte ranges when
possible, since Range supports efficient recovery from partially
failed transfers, and supports efficient partial retrieval of large
entities.

If the server supports the Range header and the specified range or
ranges are appropriate for the entity:

- The presence of a Range header in an unconditional GET modifies
what is returned if the GET is otherwise successful. In other
words, the response carries a status code of 206 (Partial
Content) instead of 200 (OK).

- The presence of a Range header in a conditional GET (a request
using one or both of If-Modified-Since and If-None-Match, or
one or both of If-Unmodified-Since and If-Match) modifies what
is returned if the GET is otherwise successful and the
condition is true. It does not affect the 304 (Not Modified)
response returned if the conditional is false.

In some cases, it might be more appropriate to use the If-Range
header (see section 14.27) in addition to the Range header.

If a proxy that supports ranges receives a Range request, forwards
the request to an inbound server, and receives an entire entity in
reply, it SHOULD only return the requested range to its client. It
SHOULD store the entire received response in its cache if that is
consistent with its cache allocation policies.

The Referer[sic] request-header field allows the client to specify,
for the server's benefit, the address (URI) of the resource from
which the Request-URI was obtained (the "referrer", although the
header field is misspelled.) The Referer request-header allows a
server to generate lists of back-links to resources for interest,
logging, optimized caching, etc. It also allows obsolete or mistyped
links to be traced for maintenance. The Referer field MUST NOT be
sent if the Request-URI was obtained from a source that does not have
its own URI, such as input from the user keyboard.

Referer = "Referer" ":" ( absoluteURI | relativeURI )

Example:

Referer: http://www.w3.org/hypertext/DataSources/Overview.html

If the field value is a relative URI, it SHOULD be interpreted
relative to the Request-URI. The URI MUST NOT include a fragment. See
section 15.1.3 for security considerations.

The Retry-After response-header field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to
be unavailable to the requesting client. This field MAY also be used
with any 3xx (Redirection) response to indicate the minimum time the
user-agent is asked wait before issuing the redirected request. The
value of this field can be either an HTTP-date or an integer number
of seconds (in decimal) after the time of the response.

The Server response-header field contains information about the
software used by the origin server to handle the request. The field
can contain multiple product tokens (section 3.8) and comments
identifying the server and any significant subproducts. The product
tokens are listed in order of their significance for identifying the
application.

Server = "Server" ":" 1*( product | comment )

Example:

Server: CERN/3.0 libwww/2.17

If the response is being forwarded through a proxy, the proxy
application MUST NOT modify the Server response-header. Instead, it
SHOULD include a Via field (as described in section 14.45).

Note: Revealing the specific software version of the server might
allow the server machine to become more vulnerable to attacks
against software that is known to contain security holes. Server
implementors are encouraged to make this field a configurable
option.

The TE request-header field indicates what extension transfer-codings
it is willing to accept in the response and whether or not it is
willing to accept trailer fields in a chunked transfer-coding. Its
value may consist of the keyword "trailers" and/or a comma-separated
list of extension transfer-coding names with optional accept
parameters (as described in section 3.6).

The presence of the keyword "trailers" indicates that the client is
willing to accept trailer fields in a chunked transfer-coding, as
defined in section 3.6.1. This keyword is reserved for use with
transfer-coding values even though it does not itself represent a
transfer-coding.

Examples of its use are:

TE: deflate
TE:
TE: trailers, deflate;q=0.5

The TE header field only applies to the immediate connection.
Therefore, the keyword MUST be supplied within a Connection header
field (section 14.10) whenever TE is present in an HTTP/1.1 message.

A server tests whether a transfer-coding is acceptable, according to
a TE field, using these rules:

1. The "chunked" transfer-coding is always acceptable. If the
keyword "trailers" is listed, the client indicates that it is
willing to accept trailer fields in the chunked response on
behalf of itself and any downstream clients. The implication is
that, if given, the client is stating that either all
downstream clients are willing to accept trailer fields in the
forwarded response, or that it will attempt to buffer the
response on behalf of downstream recipients.

Note: HTTP/1.1 does not define any means to limit the size of a
chunked response such that a client can be assured of buffering
the entire response.

2. If the transfer-coding being tested is one of the transfer-
codings listed in the TE field, then it is acceptable unless it
is accompanied by a qvalue of 0. (As defined in section 3.9, a
qvalue of 0 means "not acceptable.")

3. If multiple transfer-codings are acceptable, then the
acceptable transfer-coding with the highest non-zero qvalue is
preferred. The "chunked" transfer-coding always has a qvalue
of 1.

If the TE field-value is empty or if no TE field is present, the only
transfer-coding is "chunked". A message with no transfer-coding is
always acceptable.

The Trailer general field value indicates that the given set of
header fields is present in the trailer of a message encoded with
chunked transfer-coding.

Trailer = "Trailer" ":" 1#field-name

An HTTP/1.1 message SHOULD include a Trailer header field in a
message using chunked transfer-coding with a non-empty trailer. Doing
so allows the recipient to know which header fields to expect in the
trailer.

If no Trailer header field is present, the trailer SHOULD NOT include
any header fields. See section 3.6.1 for restrictions on the use of
trailer fields in a "chunked" transfer-coding.

Message header fields listed in the Trailer header field MUST NOT
include the following header fields:

The Transfer-Encoding general-header field indicates what (if any)
type of transformation has been applied to the message body in order
to safely transfer it between the sender and the recipient. This
differs from the content-coding in that the transfer-coding is a
property of the message, not of the entity.

If multiple encodings have been applied to an entity, the transfer-
codings MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided
by other entity-header fields not defined by this specification.

Many older HTTP/1.0 applications do not understand the Transfer-
Encoding header.

The Upgrade general-header allows the client to specify what
additional communication protocols it supports and would like to use
if the server finds it appropriate to switch protocols. The server
MUST use the Upgrade header field within a 101 (Switching Protocols)
response to indicate which protocol(s) are being switched.

Upgrade = "Upgrade" ":" 1#product

For example,

Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

The Upgrade header field is intended to provide a simple mechanism
for transition from HTTP/1.1 to some other, incompatible protocol. It
does so by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major version
number, even though the current request has been made using HTTP/1.1.
This eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly
supported protocol while indicating to the server that it would like
to use a "better" protocol if available (where "better" is determined
by the server, possibly according to the nature of the method and/or
resource being requested).

The Upgrade header field only applies to switching application-layer
protocols upon the existing transport-layer connection. Upgrade
cannot be used to insist on a protocol change; its acceptance and use
by the server is optional. The capabilities and nature of the
application-layer communication after the protocol change is entirely
dependent upon the new protocol chosen, although the first action
after changing the protocol MUST be a response to the initial HTTP
request containing the Upgrade header field.

The Upgrade header field only applies to the immediate connection.
Therefore, the upgrade keyword MUST be supplied within a Connection
header field (section 14.10) whenever Upgrade is present in an
HTTP/1.1 message.

The Upgrade header field cannot be used to indicate a switch to a
protocol on a different connection. For that purpose, it is more
appropriate to use a 301, 302, 303, or 305 redirection response.

This specification only defines the protocol name "HTTP" for use by
the family of Hypertext Transfer Protocols, as defined by the HTTP
version rules of section 3.1 and future updates to this
specification. Any token can be used as a protocol name; however, it
will only be useful if both the client and server associate the name
with the same protocol.

The User-Agent request-header field contains information about the
user agent originating the request. This is for statistical purposes,
the tracing of protocol violations, and automated recognition of user
agents for the sake of tailoring responses to avoid particular user
agent limitations. User agents SHOULD include this field with
requests. The field can contain multiple product tokens (section 3.8)
and comments identifying the agent and any subproducts which form a
significant part of the user agent. By convention, the product tokens
are listed in order of their significance for identifying the
application.

The Vary field value indicates the set of request-header fields that
fully determines, while the response is fresh, whether a cache is
permitted to use the response to reply to a subsequent request
without revalidation. For uncacheable or stale responses, the Vary
field value advises the user agent about the criteria that were used
to select the representation. A Vary field value of "*" implies that
a cache cannot determine from the request headers of a subsequent
request whether this response is the appropriate representation. See
section 13.6 for use of the Vary header field by caches.

Vary = "Vary" ":" ( "*" | 1#field-name )

An HTTP/1.1 server SHOULD include a Vary header field with any
cacheable response that is subject to server-driven negotiation.
Doing so allows a cache to properly interpret future requests on that
resource and informs the user agent about the presence of negotiation

on that resource. A server MAY include a Vary header field with a
non-cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about
the dimensions over which the response varies at the time of the
response.

A Vary field value consisting of a list of field-names signals that
the representation selected for the response is based on a selection
algorithm which considers ONLY the listed request-header field values
in selecting the most appropriate representation. A cache MAY assume
that the same selection will be made for future requests with the
same values for the listed field names, for the duration of time for
which the response is fresh.

The field-names given are not limited to the set of standard
request-header fields defined by this specification. Field names are
case-insensitive.

A Vary field value of "*" signals that unspecified parameters not
limited to the request-headers (e.g., the network address of the
client), play a role in the selection of the response representation.
The "*" value MUST NOT be generated by a proxy server; it may only be
generated by an origin server.

The Via general-header field MUST be used by gateways and proxies to
indicate the intermediate protocols and recipients between the user
agent and the server on requests, and between the origin server and
the client on responses. It is analogous to the "Received" field of
RFC 822 [9] and is intended to be used for tracking message forwards,
avoiding request loops, and identifying the protocol capabilities of
all senders along the request/response chain.

The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain. The received-protocol version is appended to
the Via field value when the message is forwarded so that information
about the protocol capabilities of upstream applications remains
visible to all recipients.

The protocol-name is optional if and only if it would be "HTTP". The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information,
it MAY be replaced by a pseudonym. If the port is not given, it MAY
be assumed to be the default port of the received-protocol.

Multiple Via field values represents each proxy or gateway that has
forwarded the message. Each recipient MUST append its information
such that the end result is ordered according to the sequence of
forwarding applications.

Comments MAY be used in the Via header field to identify the software
of the recipient proxy or gateway, analogous to the User-Agent and
Server header fields. However, all comments in the Via field are
optional and MAY be removed by any recipient prior to forwarding the
message.

For example, a request message could be sent from an HTTP/1.0 user
agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
forward the request to a public proxy at nowhere.com, which completes
the request by forwarding it to the origin server at www.ics.uci.edu.
The request received by www.ics.uci.edu would then have the following
Via header field:

Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)

Proxies and gateways used as a portal through a network firewall
SHOULD NOT, by default, forward the names and ports of hosts within
the firewall region. This information SHOULD only be propagated if
explicitly enabled. If not enabled, the received-by host of any host
behind the firewall SHOULD be replaced by an appropriate pseudonym
for that host.

For organizations that have strong privacy requirements for hiding
internal structures, a proxy MAY combine an ordered subsequence of
Via header field entries with identical received-protocol values into
a single such entry. For example,

Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

could be collapsed to

Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

Applications SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms. Applications MUST NOT combine entries which
have different received-protocol values.

The Warning general-header field is used to carry additional
information about the status or transformation of a message which
might not be reflected in the message. This information is typically
used to warn about a possible lack of semantic transparency from
caching operations or transformations applied to the entity body of
the message.

The warn-text SHOULD be in a natural language and character set that
is most likely to be intelligible to the human user receiving the
response. This decision MAY be based on any available knowledge, such
as the location of the cache or user, the Accept-Language field in a
request, the Content-Language field in a response, etc. The default
language is English and the default character set is ISO-8859-1.

If a character set other than ISO-8859-1 is used, it MUST be encoded
in the warn-text using the method described in RFC 2047 [14].

Warning headers can in general be applied to any message, however
some specific warn-codes are specific to caches and can only be
applied to response messages. New Warning headers SHOULD be added
after any existing Warning headers. A cache MUST NOT delete any
Warning header that it received with a message. However, if a cache
successfully validates a cache entry, it SHOULD remove any Warning
headers previously attached to that entry except as specified for

specific Warning codes. It MUST then add any Warning headers received
in the validating response. In other words, Warning headers are those
that would be attached to the most recent relevant response.

When multiple Warning headers are attached to a response, the user
agent ought to inform the user of as many of them as possible, in the
order that they appear in the response. If it is not possible to
inform the user of all of the warnings, the user agent SHOULD follow
these heuristics:

- Warnings that appear early in the response take priority over
those appearing later in the response.

- Warnings in the user's preferred character set take priority
over warnings in other character sets but with identical warn-
codes and warn-agents.

Systems that generate multiple Warning headers SHOULD order them with
this user agent behavior in mind.

Requirements for the behavior of caches with respect to Warnings are
stated in section 13.1.2.

This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.

110 Response is stale
MUST be included whenever the returned response is stale.

111 Revalidation failed
MUST be included if a cache returns a stale response because an
attempt to revalidate the response failed, due to an inability to
reach the server.

112 Disconnected operation
SHOULD be included if the cache is intentionally disconnected from
the rest of the network for a period of time.

113 Heuristic expiration
MUST be included if the cache heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater
than 24 hours.

199 Miscellaneous warning
The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST
NOT take any automated action, besides presenting the warning to
the user.

214 Transformation applied
MUST be added by an intermediate cache or proxy if it applies any
transformation changing the content-coding (as specified in the
Content-Encoding header) or media-type (as specified in the
Content-Type header) of the response, or the entity-body of the
response, unless this Warning code already appears in the response.

299 Miscellaneous persistent warning
The warning text MAY include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST
NOT take any automated action.

If an implementation sends a message with one or more Warning headers
whose version is HTTP/1.0 or lower, then the sender MUST include in
each warning-value a warn-date that matches the date in the response.

If an implementation receives a message with a warning-value that
includes a warn-date, and that warn-date is different from the Date
value in the response, then that warning-value MUST be deleted from
the message before storing, forwarding, or using it. (This prevents
bad consequences of naive caching of Warning header fields.) If all
of the warning-values are deleted for this reason, the Warning header
MUST be deleted as well.

The WWW-Authenticate response-header field MUST be included in 401
(Unauthorized) response messages. The field value consists of at
least one challenge that indicates the authentication scheme(s) and
parameters applicable to the Request-URI.

WWW-Authenticate = "WWW-Authenticate" ":" 1#challenge

The HTTP access authentication process is described in "HTTP
Authentication: Basic and Digest Access Authentication" [43]. User
agents are advised to take special care in parsing the WWW-
Authenticate field value as it might contain more than one challenge,
or if more than one WWW-Authenticate header field is provided, the
contents of a challenge itself can contain a comma-separated list of
authentication parameters.